fix: math in a file view -- the buffer was emptied, and the derived ring filled twice

Two defects, both reached by adding a math channel to an open waveform or a replayed
event, and both measured before and after.

**A file view has no producer, so a structural change left it empty.** `set_math_channels()`
rebuilds the channels *and* the buffer, and a rebuild drops the history: right for a stream,
whose producer refills it from the next packet, and wrong for a file, which has nothing to
refill it with. The `--open` dump read `0 samples vs 0 time slots` for *every* channel,
the file's own `sine`/`square`/`ramp`/`noise` included, though the load had reported 402 --
that is what "adding a math node breaks the plotter" was. `reload_file_view()` re-reads the
file after such a change, which is also what computes the derived channels over it: the
load's own pushes run the math once the set is in place, so `--open` with a session that
defines graphs works for the first time.

**The file path pushed into the derived rings, and so did the engine.** `push_loaded_slot()`
did not honour the rule `ChannelInfo::math` states for the packet path ("the ingest must skip
it"), so each loaded slot put a value in a derived ring and `push_math_sample()` then put the
computed one: two samples per slot, a trace at double rate -- the derived channels read
`804 samples vs 402 time slots`. Skipping them there is the whole fix.

Readings now, all equal, which is the assertion this file's own invariant makes:

    --open the file:  math x2  402 samples vs 402 time slots, y=[-1.9021, 0.0063]  (2x the sine)
                      math lp  402 vs 402, mean=-0.4900 (a real filter output, not zero)
    live:             math x2  25100 vs 25100
    replayed event:   math x2  4000 vs 4000

I first tried filtering the file's *derived columns* out by name, since an export writes
every plotted channel. It removed the double count and zeroed `math lp`, because the column
it dropped was the source a session graph reads -- so it is reverted: those columns are
legitimate data, and the fix belongs in the push, not the loader.

src/model.cpp

@@ -301,6 +301,10 @@ void ScopeModel::push_loaded_slot(const std::vector<f32>& values, f64 t, f64 abs
   const std::size_t channels = channels_.size();
   std::vector<f32> slot(channels, 0.0f);
   for (std::size_t i = 0; i < channels; ++i) {
+    // A derived channel has no column of its own: `push_math_sample()` below fills it, and pushing the
+    // file's value here as well would put *two* samples in its ring per slot -- a trace at double rate.
+    // The same rule the packet path states in `ChannelInfo::math`: the ingest must skip it.
+    if (channels_[i].math) continue;
     const f32 v = i < values.size() ? values[i] : 0.0f;
     store.values(i).push(v);
     slot[i] = v;